1. Field of the Invention
The present invention relates to a photodetection semiconductor device having a light receiving element.
2. Description of the Related Art
A CMOS image sensor includes, in general, light receiving pixel arrays as light receiving elements, in which light receiving elements are one-dimensionally or two-dimensionally arrayed. Each of the pixels includes a photodetector formed from a PN junction photodiode. Incident light into the pixels is absorbed in a semiconductor substrate to generate carriers, which recombine at a depletion layer portion of the photodiode, and then an output may be obtained as voltage or current.
Due to demands for attaining a higher speed of the device using a CMOS image sensor, a shorter read-out time of the CMOS image sensor is required. Thus, carriers stored in the photodiodes may fail to reach an electrode serving as a carrier outlet, and may appear as an afterimage at the next reading. Further, when an area of the light receiving element is increased in order to improve the sensitivity of the CMOS image sensor, a distance from an end portion of the light receiving element to the electrode serving as the carrier outlet is increased to be longer than a distance of moving carrier through diffusion, resulting in afterimages.
As one of improvement measures for preventing the generation of afterimages, there is proposed a method of reducing afterimages, involving providing electric potential gradient to light receiving elements to cause drift, thereby increasing the moving speed of carriers (for example, see Japanese Patent Application Laid-open No. 2002-237614).
However, when the potential gradient is provided into the light receiving elements, a period for eliminating a potential difference is necessary, and hence the demands for attaining a higher speed cannot be satisfied.